The present invention is concerned with processes for the preparation, isolation and purification of certain L-aspartyl-D-alaninamides and L-aspartyl-D-serinamides having a branched organic radical as substituent on the amide nitrogen. These processes in all instances include crystallization of said amides as acid addition salts with aromatic sulfonic acids. These salts per se, in anhydrous or hydrated form, are also encompassed by the present invention.
The aforementioned amides are potent synthetic sweeteners. The D-alaninamide derivatives are fully disclosed as to preparation and utility in European Patent Document No. 34,876, published Sept. 2, 1981. The D-serinamide derivatives, prepared and tested by the same methods, are described in copending U.S. patent application, Ser. No. 276,243, filed June 26, 1981, for "Branched Amides of L-Aspartyl-D-Amino Acid Dipeptides" to Brennan and Hendrick. While both of these patent documents refer in general to pharmaceutically-acceptable acid addition salts, there is no mention of sulfonic acid salts.
In both the D-alanine and D-serine series, two of the earlier preferred methods of synthesis involve multistep sequences, generally requiring protection and subsequent deprotection of amine and/or carboxyl groups. The present crystallization process offers an advantageous method for purification of said dipeptide sweeteners when prepared by one of these earlier preferred methods.
A more particular and surprising advantage of the present invention is in the "one-step" synthesis presented earlier as a third preferred method in both the D-alanine and D-serine series. In this latter process L-aspartic N-thiocarboxyanhydride is reacted with a suitably subsituted D-alaninamide or D-serinamide to yield the desired dipeptide amide directly. Disadvantages of the latter process are the tendency to co-produce isomeric D-aspartyl-D-amino acid amide and tripeptide (L-aspartyl-L-aspartyl-D-amino acid amide), as well as traces of foul smelling sulfur compounds generally difficult to remove (cf. Vinick, U.S. Pat. No. 4,238,392, December 1980, whereby alkali metal periodates are employed to deodorize L-aspartyl-L-phenylalanine alkyl esters prepared by use of L-aspartic acid N-thiocarboxyanhydride). Surprisingly, the present crystallization process urges out the tripeptide and the diastereomeric D-aspartyl-D-amino acid amide (even when the latter is present in high concentration, e.g. 10%). Furthermore, odiferous sulfur compounds are reduced in level or eliminated. Thus the present invention renders an earlier "third preferred process" now a particularly preferred process.
The present invention also encompasses another, particularly preferred, "one-step" process which is novel to the synthesis of the present compounds. An acid addition salt of L-aspartic acid anhydride is coupled with the same suitably substituted D-alaninamide or D-serinamide. In this process there is no odor problem such as that noted above; however, a large amount (generally an amount equal to about one-half that of the desired product) of the beta-L-aspartyl isomer is formed in this process, as well as some tripeptide (L-aspartyl-L-aspartyl-D-alaninamide or -serinamide). Surprisingly both the beta-isomer and the tripeptide are removed by the present crystallization process.
Ariyoshi et al. [Bull. Chem. Soc. Japan, 46, pp. 1893-1895 (1975)] have described the preparation of aspartame (L-aspartyl-L-phenylalanine methyl ester) from L-aspartic acid anhydride hydrochloride and methyl L-phenylalanate hydrochloride. Partial purification was achieved by crystallization of hydrochloride salt, but column chromatography was employed for the ultimate purification of the desired product, not the selective crystallization of the present invention, which as noted below, is not applicable to aspartame.
Although certain aromatic carboxylic acid salts have been used to separate aspartame from its beta-L-aspartyl isomer (Ariyoshi and Sato, U.S. Pat. No. 3,673,026, July 1972), the present result is all the more suprising in view of the facts that (1) aspartame does not crystallize from water in the presence of p-toluenesulfonic acid, i.e. under the present conditions; and (2) the aromatic carboxylic acid salts of Ariyoshi and Sato are not operative with the present dipeptide amide sweeteners.